Na+,K+-ATPase activity of the subcellular fractions of the rat brain in aging

The Na+, K+-ATPase activity in the homogenate and in subcellular fractions of different parts of the brain of adult and old rats was studied in comparison. The content of cholesterol in the above fractions was also determined. In old age the Na+, K+-ATPase activity in the homogenate and microsomal fraction of the cerebral hemispheres' cortex decreases, while the Mg2+-ATPase activity in the cortex microsomal fraction increases. The age-related Na+, K+- and Mg2+-ATPase activity in the myelin of the stem in the synaptic plasma membranes of hemispheres and the brain stem remains unchanged whereas in the myelin fraction of hemispheres it grows. The content of cholesterol in the brain of old rats as compared with adult ones increases in the microsomal fraction and remains unchanged in synaptic membranes. The possible role of age-related modification of lipid component of plasma membranes in the above changes of Na+, K+-ATPase activity is discussed.     

Functional Significance of the Effects of Neurotransmitters on the Na+,K+-ATPase System

The aim of the present experiments was to study the effects of the neurotransmitters acetylcholine, noradrenaline, 5-hydroxytryptamine, and dopamine on the Na+,K+-ATPase of rat brain synaptosomal fractions. It is shown that dopamine at low concentrations specifically inhibits the Na+,K+-ATPase of synaptic membranes from the brain regions rich in dopaminergic endings, but has no effect on the synaptosomal Na+,K+-ATPase from the other parts of brain. Acetylcholine and noradrenaline have similar specific effects on Na+,K+-ATPase from cholinergic and adrenergic synaptosomes. The Na+,K+-ATPase of synaptic membranes from the different brain regions, characterised by different distributions of cholinergic, adrenergic, and 5-hydroxytryptaminergic endings, show different reactions with neurotransmitters. These data indicate a functional significance of the effects of the neurotransmitters on the synaptosomal Na+,K+-ATPase.     

The effect of antimembrane testicular antibodies on Na+, K(+)-ATPase activity in the testicles of rats of different ages]

The effect of large and small doses of rabbit antibodies specific to plasma membranes of the rat testicle cells has been studied in the experiments on Wistar rats of three age groups (preadolescent--aged 20 days, puberal--aged 5-7 months and old--aged 24-26 months). It is stated that incubation of plasma membranes by IgG fraction isolated from antimembrane testicular serum (IgG-ATCSm) in a large dose (43 g of protein G per 125 g of protein of membrane fraction) caused statistically reliable inhibition of Na+, K(+)-ATPase activity in the membranes of testicle cells of puberal and old rats. Preadolescent rats exhibit only a tendency to decrease the activity of this enzyme. Incubation of plasma membranes of testicle cells in rats of different age by small doses of IgG-ATCSm (0.43 g of protein G per 125 g of membrane protein) induced a statistically reliable increase of Na+, K(+)-ATPase activity in puberal and old animals and its slight increase in preadolescent rats. The IgG fraction isolated from normal rabbit serum (IgG-NRS) exerted a less pronounced effect on Na+, K(+)-ATPase activity parallel with retention of a tendency to a decrease of activity under the influence of large doses of the drug and to an increase with introduction of small doses.     

Fluorescence Polarization Analysis, Lipid Composition, and Na+, K+-ATPase Kinetics of Synaptosomal Membranes in Feline GM1 and GM2 Gangliosidosis

Neurochemical studies were performed on synaptosomal membranes from cats with GM1 or GM2 gangliosidosis to examine possible mechanisms of neuronal dysfunction in these disorders. The basic hypothesis tested was that deficient ganglioside catabolism causes increased ganglioside content of synaptosomal plasma membrane which in turn disrupts normal function. Fluidity characteristics of synaptosomal membranes were examined using fluorescence polarization. Results showed markedly reduced membrane fluidity in both GM1 and GM2 gangliosidosis. These results were supported by a second study which revealed that isolated synaptosomal membranes of GM1 gangliosidosis cats had a 24-fold increase in total ganglioside content caused predominantly by excess GM1, a 2.3-fold increased cholesterol content, and a 1.4-fold increased phospholipid content. Finally, kinetic analysis of synaptosomal plasma membrane Na+,K+-ATPase from cats with GM1 gangliosidosis showed negligible differences in kinetic parameters compared with controls. Thus, the enzyme appeared protected from the global membrane changes in fluidity and composition. These observations provide evidence for a pathogenetic mechanism of neuronal dysfunction in the gangliosidoses while demonstrating protection of certain vital functional components, such as Na+,K+-ATPase.     

Nantenine and papaverine differentially modify synaptosomal membrane enzymes.

Papaverine (1-[(3,4-Dimethoxyphenyl) methyl]-6,7-dimethoxyisoquinoline) and nantenine (O-methyldomesticine) are chemically related isoquinoline alkaloids displaying similar dose-dependent sedative or convulsant effects, but seem to act differentially on synaptosomal membrane enzymes. Na+, K+-, Mg2+- and Ca2+-ATPase activities were inhibited by nantenine but not by papaverine, whereas acetylcholinesterase activity remained unchanged by nantenine but slightly enhanced by papaverine. Nantenine inhibited roughly both 20-50% Ca2+- and Mg2+-ATPase activities but 40-90% Na+, K+-ATPase activity. Kinetic analysis indicated that nantenine interacts with the substrate ATP for Ca2+-ATPase activity but that it competes with K+ for Na+, K+-ATPase activity. Given the roles of Na+, K+-ATPase and Ca2+-ATPase in cation transport and [Ca2+]i regulation, respectively, the inhibitory effect of nantenine upon these enzymes may explain its convulsant effect though not its sedative activity. The sedative action of both nantenine and papaverine is hardly attributable to an effect on the synaptosomal membrane enzymes assayed.     

Na+, K+-ATPase activity and serotonin transport in the rat brain synaptosomes fractions after acute 1,2-dichloroethane intoxication and nicotinamide administration

Alterations of Na+,K+-ATPase activity and serotoninergic system functioning were investigated in brain synaptosomes fractions of rats under experimental acute 1,2-dichloroethane (DChE) intoxication. It was shown that Na+,K+-ATPase activity was markedly increased (by 41,8%) in a period of 24 h after DChE intoxication and decreased (by 27%) after 48 h intoxication. The level of [2-14C]-serotonin uptake by synaptosomes was progressively diminished after 24 and 48 h after DChE injection whereas the activity of monoamine uptake proved to be unchanged. Nicotinamide (200 mg/kg of body weight) was administered to rats subjected to DChE 1, 24 and 36 h after poisoning. The treatment of rats with nicotinamide resulted in some normalization of brain synaptosomal Na+, K+-ATPase activity and serotonin uptake controlled at 48 h after DChE intoxication.     

The aging of a brain soluble fraction modifies its effect on the activity of neuronal Na+, K+-ATPase

In previous work we presented evidence showing that a brain soluble fraction was necessary to observe the stimulation of membrane Na+,K+-ATPase activity by catecholamines. Preliminary experiments suggested to us that the soluble fraction by itself was able to modify this enzyme activity. In the present study we have assayed the activity of synaptosomal Na+,K+-ATPase in the presence of a soluble fraction (aqueous supernatant after 100,000 g 30 min) prepared from rat cerebral cortex. The soluble fraction was used at different times after its preparation and different conditions in the incubation period previous to the enzyme assay were tested. It was observed that the enzyme activity increased 70% in the presence of a "0 min" soluble fraction. This effect was not found: a) in the presence of a "30 min" soluble fraction or b) when the membranes plus a "0 min" soluble fraction were incubated for 30 min (15 min at 37 degrees C + 15 min at 0 degree C) before the ATPase assay. In the presence of a "60 min" or "24 h" soluble fraction Na+,K+-ATPase activity was inhibited 50%. Results obtained indicate that Na+,K+-ATPase activity of synaptosomal membranes can be stimulated, inhibited or unchanged, depending on the aging of the soluble fraction.     

Subcellular distribution of K+-pNPPase (Na+,K+-ATPase) in the Golgi apparatus of developing rat cortical neurons

The distribution of K+-pNPPase (Na+,K+-ATPase) activity in the compartments of the Golgi apparatus in neurons of the cerebral cortex of young and adult Wistar rats was studied by ultrastructural cytochemistry. In adult rats, mainly the cis-most cisterna was associated with reaction deposits. In 10- and especially in 15-day-old rats, not only the cis-cisternae, but the cis- and trans-Golgi, as well as components of the Golgi stack, also revealed K+-pNPPase activity. The dynamic changes of K+ -pNPPase localization in the compartments of the neuronal Golgi complexes were discussed with respect to the biochemical evidence concerning the building, assembly and processing of Na+,K+-ATPase as plasma membrane glycoprotein. It was suggested that the high activity in the Golgi complexes seen in 15-day-old rats has to be associated with the advancing myelinization in this period and the necessity of Na+,K+-ATPase equipment of nodes of Ranvier.     

A quantitative immunocytochemical study of Na+,K+-ATPase in rat hepatocytes after STZ-induced diabetes and dietary fish oil supplementation.

Because diabetes causes alterations in hepatic membrane fatty acid content, these changes may affect the Na+,K+-ATPase. In this study we documented the effects of streptozotocin (STZ)-induced diabetes on hepatic Na+,K+-ATPase catalytic alpha1-subunit and evaluated whether these changes could be normalized by fish oil supplementation. Two groups of diabetic rats received fish oil or olive oil supplementation. Both groups had a respective control group. We studied the localization of catalytic alpha1-subunit on bile canalicular and basolateral membranes using immunocytochemical methods and confocal laser scanning microscopy, and the Na+, K+-ATPase activity, membrane fluidity, and fatty acid composition on isolated hepatic membranes. A decrease in the alpha1-subunit was observed with diabetes in the bile canalicular membranes, without changes in basolateral membranes. This decrease was partially prevented by dietary fish oil. Diabetes induces significant changes as documented by enzymatic Na+,K+-ATPase activity, membrane fluidity, and fatty acid content, whereas little change in these parameters was observed after a fish oil diet. In conclusion, STZ-induced diabetes appears to modify bile canalicular membrane integrity and dietary fish oil partly prevents the diabetes-induced alterations.     

Regulation of rat brain (Na+ +K+)-ATPase activity by cyclic AMP

The interaction between the (Na+ +K+)-ATPase and the adenylate cyclase enzyme systems was examined. Cyclic AMP, but not 5'-AMP, cyclic GMP or 5'-GMP, could inhibit the (Na+ +K+)-ATPase enzyme present in crude rat brain plasma membranes. On the other hand, the cyclic AMP inhibition could not be observed with purified preparations of (Na+ +K+)-ATPase enzyme. Rat brain synaptosomal membranes were prepared and treated with either NaCl or cyclic AMP plus NaCl as described by Corbin, J., Sugden, P., Lincoln, T. and Keely, S. ((1977) J. Biol. Chem. 252, 3854-3861). This resulted in the dissociation and removal of the catalytic subunit of a membrane-bound cyclic AMP-dependent protein kinase. The decrease in cyclic AMP-dependent protein kinase activity was accompanied by an increase in (Na+ +K+)-ATPase activity. Exposure of synaptosomal membranes containing the cyclic AMP-dependent protein kinase holoenzyme to a specific cyclic AMP-dependent protein kinase inhibitor resulted in an increase in (Na+ +K+)-ATPase enzyme activity. Synaptosomal membranes lacking the catalytic subunit of the cyclic-AMP-dependent protein kinase did not show this effect. Reconstitution of the solubilized membrane-bound cyclic AMP-dependent protein kinase, in the presence of a neuronal membrane substrate protein for the activated protein kinase, with a purified preparation of (Na+ +K+)-ATPase, resulted in a decrease in overall (Na+ +K+)-ATPase activity in the presence of cyclic AMP. Reconstitution of the protein kinase alone or the substrate protein alone, with the (Na+ +K+)-ATPase has no effect on (Na+ +K+)-ATPase activity in the absence or presence of cyclic AMP. Preliminary experiments indicate that, when the activated protein kinase and the substrate protein were reconstituted with the (Na+ +K+)-ATPase enzyme, there appeared to be a decrease in the Na+-dependent phosphorylation of the Na+-ATPase enzyme, while the K+-dependent dephosphorylation of the (Na+ +K+)-ATPase was unaffected.     

Activities of enzymes from canine kidney plasma membranes under effects of polyene antibiotics in vivo and in vitro]

The effects of amphotericin B drug containing sodium deoxycholate (DOC) and those of DOC and nistatin on the activities of Na+, K+-ATPase and 5'-nucleotidase of canine kidney plasma membranes were studied. It was found that the activities of Na+, K+-ATPase and 5'-nucleotidase were markedly inhibited only after intravenous injection of amphotericin B, whereas the other agents tested caused no changes in the enzyme activities. Similar results were obtained in vitro. In the presence of amphotericin B the activity of Na+, K+-ATPase was noticeably inhibited already at the antibiotic concentration of 0,1 mkg per mg of membrane protein. It was found that the injection of amphotericin B, DOC and nistatin did not qualitatively or quantitatively affect the phospholipid composition of the plasma membranes. This is indicative of the lack of correlation between the enzyme activities and changes in the phospholipid composition of the plasma membranes under effects of amphotericin B. The pyrimidine derivative--amygluracyl--markedly removes the inhibiting effect of amphotericin B on the enzyme activity of plasma membranes.     

Localization of Na+,K+-ATPase alpha-subunit to the sinusoidal and lateral but not canalicular membranes of rat hepatocytes

Controversy has recently developed over the surface distribution of Na+,K+-ATPase in hepatic parenchymal cells. We have reexamined this issue using several independent techniques. A monoclonal antibody specific for the endodomain of alpha-subunit was used to examine Na+,K+-ATPase distribution at the light and electron microscope levels. When cryostat sections of rat liver were incubated with the monoclonal antibody, followed by either rhodamine or horseradish peroxidase-conjugated goat anti-mouse secondary, fluorescent staining or horseradish peroxidase reaction product was observed at the basolateral surfaces of hepatocytes from the space of Disse to the tight junctions bordering bile canaliculi. No labeling of the canalicular plasma membrane was detected. In contrast, when hepatocytes were dissociated by collagenase digestion, Na+,K+-ATPase alpha-subunit was localized to the entire plasma membrane. Na+,K+-ATPase was quantitated in isolated rat liver plasma membrane fractions by Western blots using a polyclonal antibody against Na+,K+-ATPase alpha-subunit. Plasma membranes from the basolateral domain of hepatocytes possessed essentially all of the cell's estimated Na+,K+-ATPase catalytic activity and contained a 96-kD alpha-subunit band. Canalicular plasma membrane fractions, defined by their enrichment in alkaline phosphatase, 5' nucleotidase, gamma-glutamyl transferase, and leucine aminopeptidase had no detectable Na+,K+-ATPase activity and no alpha-subunit band could be detected in Western blots of these fractions. We conclude that Na+,K+-ATPase is limited to the sinusoidal and lateral domains of hepatocyte plasma membrane in intact liver. This basolateral distribution is consistent with its topology in other ion-transporting epithelia.     

Inactivation of Na+, K+ -ATPase from cattle brain by sodium fluoride

The influence of the physiological ligands and modifiers on the plasma membrane Na+, K+ -ATPase from calf brain inactivation by sodium fluoride (NaF) is studied. ATP-hydrolyzing activity of the enzyme was found to be more stable as to NaF inhibition than its K+ -pNPPase activity. The activatory ions of Na+, K+ -ATPase have different effects on the process of the enzyme inhibition by NaF. K+ intensifies inhibition, but Na+ does not affect it. An increase of [Mg2+free] in the incubation medium (from 0.5 to 3.0 mM) rises the sensitivity of Na+, K+ -ATPase to NaF inhibition. But an increase of [ATP] from 0.3 to 1.5 mM has no effect on this process. Ca and Mg ions modify Na+, K+ -ATPase inhibition by fluoride differently. Ca2+free levels this process, and Mg2+free on the contrary increases it. In the presence of Ca ions and in the neutral-alkaline medium (pH 7.0-8.5) the recovery of activity of the transport ATPase inhibited by-NaF takes place. Sodium citrate also protects both ATP-hydrolizing and K-pNPPase activity of the Na+, K+ -ATPase from NaF inhibition. Under the modifing membranous effects (the treatment of plasma membranes by Ds-Na and digitonin) the partial loss of Na+, K+ -ATPase sensitivity to NaF inhibition is observed. It is concluded that Na+, K+ -ATPase inactivation by NaF depends on the influence of the physiological ligands and modifiers as well as on the integrity of membrane structure.     

Effect of amiloride on the activity of membrane-bound and soluble Na+-,K+-ATPase preparations

Amiloride (8 X 10(-4), an inhibitor of sodium channels of nonexcited membranes, inhibits the activity of Na+,K+-ATPase in the kidney cortex homogenate as well as that of the partially purified membrane-bound and lubrol-soluble Na+,K+-ATPase preparations from the cattle brain. Inhibition of Na+,K+-ATPase from different organs of various animals by amiloride, a blocker of sodium channels, indicates similarity of the molecular organization of the Na+-recognizing component both of sodium channels and sodium centres of Na+,K+-ATPase.     

Proteolytic digestion of Cl(-)-ATPase in rat brain synaptosomes

Upon tryptic digestion of synaptosomes, ATPase activities decreased in the order of Cl(-)-ATPase greater than or equal to Na+,K+-ATPase greater than anion-insensitive Mg2+-ATPase. Upon synaptosome treatment with hypotonic solution, Cl(-)-ATPase or anion-insensitive Mg2+-ATPase was slightly inactivated, while Na+,K+-ATPase underwent a much larger degree of inactivation. ATP-Mg inhibited the ATPase digestion in the hypotonic-solution-treated synaptosomes in a concentration-dependent manner, but not in the untreated synaptosomes. These results suggest that trypsin-digestible site of Cl(-)-ATPase are present on both sides of the synaptosomal plasma membrane, and the ATP-Mg binding site of the enzyme is located on the inner surface of the membrane.     

Salt intake and intestinal dopaminergic activity in adult and old Fischer 344 rats

We have earlier shown that the renal dopaminergic system failed to respond to high salt (HS) intake in old (24-month-old) Fisher 344 rats (Hypertension 1999;34:666-672). In the present study, intestinal Na+,K+-ATPase activity and intestinal dopaminergic tonus were evaluated in adult and old Fischer 344 rats during normal salt (NS) and HS intake. Basal intestinal Na+,K+-ATPase activity (nmol Pi/mg protein/min) in adult rats (142+/-6) was higher than in old Fischer 344 rats (105+/-7). HS intake reduced intestinal Na+,K+-ATPase activity by 20% (P<0.05) in adult, but not in old rats. Dopamine (1 microM) failed to inhibit intestinal Na+,K+-ATPase activity in both adult and old Fischer 344 rats (NS and HS diets). In adult animals, co-incubation of pertussis toxin with dopamine (1 microM) produced a significant inhibitory effect in the intestinal Na+,K+-ATPase activity. L-DOPA and dopamine tissue levels in the intestinal mucosa of adult rats were higher (45+/-9 and 38+/-4 pmol/g) than those in old rats (27+/-9 and 14+/-1 pmol/g). HS diet did not change L-DOPA and DA levels in both adult and old rats. DA/L-DOPA tissue ratios, an indirect measure of dopamine synthesis, were higher in old (1.1+/-0.2) than in adult rats (0.6+/-0.1). Aromatic L-amino acid decarboxylase (AADC) activity in the intestinal mucosa of old rats was higher than in adult rats. HS diet increased the AADC activity in adult rats, but not in old rats. It is concluded that intestinal dopaminergic tonus in old Fisher 344 rats is higher than in adult rats and is accompanied by lower basal intestinal Na+,K+-ATPase activity. In old rats, HS diet failed to alter the intestinal dopaminergic tonus or Na+,K+-ATPase activity, whereas in adult rats increases in AADC activity were accompanied by decreases in Na+,K+-ATPase activity. The association between salt intake, increased dopamine formation and inhibition of Na+,K+-ATPase at the intestinal level was not as straightforward as that described in renal tissues.     

Comparative study of calixarene effect on Mg2+ -dependent ATP-hydrolase enzymatic systems from smooth muscle cells of the uterus

Investigation the influence of calyx[4]arenes C-90, C-91, C-97 and C-99 (codes are indicated) on the enzymatic activity of four functionally different Mg2+ -dependent ATPases from smooth muscle of the uterus: actomyosin ATPase, transporting Ca2+, Mg2+ -ATPase, ouabain-sensible Na+, K+ -ATPase and basal Mg2+ -ATPase. It was shown that calixarenes C-90 and C-91 in concentration 100 microM act multidirectionally on the functionally different Mg2+ -dependent ATP-hydrolase enzymatic systems. These compounds activate effectively the actomyosin ATPase (Ka = 52 +/- 11 microM [Ukrainian character: see text] 8 +/- 2 microM, accordingly), at the same time calixarene C-90 inhibited effectively activity of transporting Ca2+, Mg2+ -ATPase of plasmatic membranes (I(0,5) = 34.6 +/- 6.4 microM), but influence on membrane-bound Na+, K+ -ATPase and basal Mg2+ -ATPase. Calixarene C-91 reduce effectively basal Mg2+ -ATPase activity, insignificantly activating Na+, K+ -ATPase but has no influence on transporting Ca2+, Mg2+ -ATPase activity of plasmatic membranes. Calixarenes C-97 and C-99 (100 microM), which have similar structure, have monodirectional influence on activity of three functionally different Mg2+-dependent ATPases of the myometrium: actomyosin ATPase and two ATPases, that related to the ATP-hydrolases of P-type--Ca2+, Mg2+ -ATPase and Na+, K+ -ATPase of plasmatic membranes. Basal Mg2+ -ATPase is resistant to the action of these two connections. Results of comparative experiments that were obtained by catalytic titration of calixarenes C-97 and C-99 by actomyosin ATPase (I(0,5) = 88 +/- 9 and 86 +/- 8 microM accordingly) and Na+, K+ -ATPase from plasmatic membranes (I(0,5) = 33 +/- 4 and 98 +/- 8 nM accordingly) indicate to the considerably more sensitiveness of Na+, K+ -ATP-ase to these calixarenes than ATPase of contractile proteins. Thus, it is shown that calixarenes have influence on activity of a number of important enzymes, involved in functioning of the smooth muscle of the uterus and related to energy-supplies of the process of the muscle contracting and support of intracellular ionic homeostasis. The obtained results can be useful in further researches, directed at the use of calixarenes as pharmaceutical substance, able to normalize the contractile function of the uterus at some pregnancy pathologies in women's.     

Calcium-Activated ATPases in Presynaptic Nerve Endings

We studied the properties of calcium-activated ATPases present in preparations of isolated presynaptic nerve ending (synaptosome) and its subfractions from mouse brain. ATPase activity in the preparation was stimulated by Ca2+ and by Mg2+, but not by Na+ and K+, when each was added alone. The substrate specificities were found to be similar. The ATPases hydrolyzed only the high-energy phosphate bond and similar activity was exhibited for all nucleoside triphosphates tested (ATP, CTP, GTP, UTP). Moreover, the enzymes were insensitive to mitochondrial markers and to ouabain, but were inhibited by La3+. La3+ produced uncompetitive inhibition of Ca2+-ATPase in intact synaptosomes. Inhibition by La3+ was greatly increased after lysis of the synaptosomes, suggesting that the active sites of the enzymes may be on the cytosolic face of the membranes. The Ca2+-ATPase activity in synaptosomes was increased by increasing concentrations of external K+, suggesting that Ca2+ influx may be involved The Ca2+-ATPase in synaptosomal plasma membranes and synaptic vesicles had higher specific activities than those of intact synaptosomes and were activated, both in the presence and the absence of Mg2+, by Ca2+ concentrations approximating the intracellular level (10(-7) M). It is concluded that the nonmitochondrial synaptosomal Ca2+-ATPase may play an important role in the regulation of intracellular Ca2+.     

Age-related changes in [3H] ouabain binding to synaptic plasma membranes isolated from mouse brains.

Age-related changes in ouabain binding to synaptic plasma membranes isolated from cerebral cortices of C57BL/6 mice were investigated to examine whether the density of Na+, K(+)-ATPase decreases with advancing age. Specific binding of [3H]ouabain did not change until around 20 months of age, but a 22% decrease in binding was found in the late senescent stage (29 months). Scatchard analysis of the binding revealed that the maximum number of binding sites (Bmax) was lower in aged mice, while the binding affinity (Kd) for ouabain receptor remained unchanged with aging. These results indicate that the density of Na+,K(+)-ATPase enzyme sites in the plasma membranes of brain synapses decreases in aged mice. Since the activity of Na+,K(+)-ATPase has been found to start declining at a much earlier stage [Tanaka, Y. & Ando, S. (1990) Brain Res. 506, 46-52; Ando, S. & Tanaka, Y. (1990) Gerontology 36, 10-14] than that at which the decrease of Bmax is manifested, at least two mechanisms may underlie the age-related decrease of the enzyme activity. We speculate that the lipid microenvironment which regulates the enzyme activity starts to change at the early stage of senescence, followed by the decrease in the enzyme content in the later stage, that is, both changes cooperatively diminish the Na+,K(+)-ATPase activity in senescence.     

An Activation of Synaptosomal Na+, K+-ATPase by a Novel Dibenzoxazepine Derivative (BY-1949) in the Rat Brain: Its Functional Role in the Neurotransmitter Uptake Systems

In search of factors mitigating the final outcome of ischemic and epileptic brain damage, we tested a novel dibenzoxazepine derivative (BY-1949), as the compound has been shown to be effective under these two conditions. First, using rat brain, we assessed whether or not BY-1949 affects the Na+,K(+)-ATPase activity. Although in vitro applications of either BY-1949 or its three major metabolites did not cause any apparent effects, both acute and chronic oral administrations of the compound (10 mg/kg) invariably increased the Na+,K(+)-ATPase activity in the synaptosomal plasma membranes by increasing Vmax values. Second, it was shown by this study that the drug treatment caused marked increases in the uptake of both glutamic acid and gamma-aminobutyric acid into the synaptosomes. These results suggest that the activity against ischemic/epileptic brain damage by BY-1949 is explicable, at least partly, in terms of improvement of ionic derangements across the neural membranes via Na+,K(+)-ATPase activation.